This invention relates generally to carburetion systems for supplying a fuel-air mixture to an internal combustion engine, and more particularly to a system in which the effective parameters of a Venturi tube in a carburetor are controlled to optimize the shape and area ratios of the tube under varying conditions of operation.
The function of a carburetor is to produce the fuel-air mixture needed for the operation of an internal combustion engine. In the carburetor, the fuel is distributed in the form of tiny droplets in a stream of air, the droplets being vaporized as a result of heat absorption on the way to the combustion chamber whereby the mixture is rendered inflammable.
In a conventional carburetor, air flows into the carburetor through a Venturi tube which is generally circular in shape. The reduction in pressure at the Venturi throat causes fuel to flow from a float chamber in which the fuel is stored through a fuel jet into the air stream, the fuel being atomized because of the difference between air and fuel velocities.
The behavior of an internal combustion engine in terms of operating efficiency, fuel economy and the emission of pollutants is directly affected by the fuel-air ratio of the combustible charge. Under ideal circumstances, the engine should at all times burn 14.5 parts of air to one part of fuel to satisfy the stoichiometric air-to-fuel ratio. But in actual operation, this ratio varies as a function of operating speed and is affected by changes in load and temperature.
To obtain maximum economy, the fuel-to-air ratio in the mixture should be maintained within close tolerances in all modes of operation, such as "idle" while standing still, "slow-speeds" up to about 20 miles an hour, "cruising speeds" and "high speeds." The conventional practice is to provide an accelerating pump system to furnish an extra charge of fuel for quick bursts of speed, a choke system to enrich the mixture for starting a cold engine and a throttle by-pass jet for idle and slow speed.
Another reason why the maintenance of a steady fuel-to-air ratio is important is that the emission of pollutants is in large measure governed by this ratio. Thus, when the mixture is relatively low in air, carbon monoxide is produced, and when the ratio is excessively rich in fuel, unburned hydrocarbons are emitted in the exhaust.
A major problem encountered in carburetion is to secure the correct amount of suction around the needle valve at slow engine speeds and yet allow enough air to enter at high engine speeds to maintain the desired ratio of air and fuel. Venturi size must, of necessity, represent a compromise for both high and low speed operation. Because the maximum power an engine can develop is limited by the amount of air it can breathe in, the Venturi size should offer minimum resistance to the larger volume of air flowing at high engine speed. On the other hand, a small Venturi is desirable at low engine speeds to afford sufficient air velocity for controllable fuel metering and good fuel atomization.
The modern approach to this problem is the use of two or more Venturis arranged in series. The multiple Venturi design serves two purposes: First, the added Venturis build up air velocity in the smaller primary Venturi, thereby augmenting the force available at the main nozzle for drawing and atomizing fuel. Second, air by-passing the primary Venturi forms an air cushion around the rich mixture discharged by the Venturi, tending to improve mixture distribution by preventing fuel from engaging the carburetor walls. Idle or very slow speed is invariably served by an auxiliary jet around the edge of the throttle plate.
However, the typical modern carburetor requires a series of additional jets and pumping systems that cut in and out as the carburetor velocity increases and decreases above and below average speed, and as the engine operation passes through successive operating modes of acceleration, cruising, high speed and deceleration. Idle or very slow speed operations both rely on an idle jet arrangement at the closed position of the butterfly throttle valve. The actions of these auxiliary devices give rise to large fluctuations in the air-fuel ratio and thereby adversely affect fuel economy.
But fuel economy is not the only reason for maintaining a steady air-to-fuel ratio; for, as pointed out in Business Week (June 21, 1976), though a new catalytic converter is available which is adapted to limit the emission of hydrocarbons, carbon monoxide and nitrogen oxides, "A steady ratio (air-to-fuel) is crucial to the new converter because it must simultaneously harbor conflicting chemical reactions." As pointed out in this article, "in actual operation, the ratio fluctuates with acceleration and deceleration."
Although fuel-air mixtures may be introduced to the combustion chambers of an engine by means other than carburetors, as by fuel injection, supercharging and other expedients, none of these is comparable in effectiveness with the Venturi principle for efficient atomization of volatile fuels.
Attempts have heretofore been made to provide variable-Venturi carburetors to tailor the air-fuel supply to changing engine conditions. Thus U.S. Pat. Nos. 2,066,544; 3,659,572 and 3,778,041 show various embodiments of a variable-Venturi carburetor. But the arrangements disclosed in these patents are incapable of varying the effective parameters of a Venturi tube so as to maintain the optimum shape and area ratios of the tube throughout the operating range and to properly locate the fuel nozzles or jets in a continuously changing Venturi throat.
The throat of a Venturi, as this term is used herein, refers to that cross-section of the air-flow passage in the Venturi that is either the smallest or through which the air flow velocity is greatest, or conversely in which the static pressure is lowest.